1. Field of Invention
The invention relates to a piezoelectric transducer and an ink ejector using a piezoelectric transducer.
2. Description of Related Art
A piezoelectric ink ejector has been conventionally proposed for a printhead. In a drop-on-demand ink ejector, a piezoelectric transducer deforms to change the volume of an ink channel containing ink. Ink in the ink channel is ejected from a nozzle when the volume is reduced, while ink is drawn into the ink channel when the volume is increased. Typically, a number of such ink ejecting mechanisms are disposed adjacent to each other, and ink is selectively ejected from an ink ejecting mechanism located in a particular position to form desired characters and images.
In a conventional piezoelectric ink ejector, one piezoelectric transducer is used for each ink ejecting mechanism. In that case, if many ink ejecting mechanisms are clustered to print an image over a wide range at high resolution, the ink ejector becomes complicated in structure and expensive to manufacture. In addition, it is hard to downsize each ejecting mechanism because the piezoelectric transducer cannot be made smaller due to machining constraints. Thus, the resolution is limited in such an ink ejector.
To address the forgoing problems, a single piezoelectric transducer disposed across a plurality of ink channels has recently been proposed for a piezoelectric ink ejector. A portion of the single piezoelectric transducer corresponding to a particular ejecting mechanism is locally deformed. Such a piezoelectric ink ejector is disclosed in U.S. Pat. No. 5,266,964. A piezoelectric ink ejector that has the same operation principle as the ink ejector disclosed in that patent is shown in FIG. 16.
FIG. 16 is a sectional view of a conventional piezoelectric ink ejector 501. As shown in FIG. 1, the piezoelectric ink ejector 501 includes a piezoelectric transducer 500 disposed across a plurality of ink chambers 60 to change the volume of the ink chambers 60. The piezoelectric transducer 500 is formed by laminating piezoelectric ceramic layers 510 while sandwiching inner-electrodes 530, 540 therebetween.
The piezoelectric ceramic layers 510 are polarized in directions shown by arrows 550, parallel to the laminating direction. Inner center electrodes 530 are placed at the center of each ink channel 60, and inner side electrodes 540a, 540b are placed on both sides of each ink channel 60.
When an ink droplet is ejected from an ink channel 60a based on predetermined print data, a drive voltage is applied to the side inner electrodes 540a, 540b and to the inner center electrodes 530a. In this case, the inner center electrodes 530a has a positive potential while the inner side electrodes 540a, 540b are grounded. Accordingly, electrical fields are generated in areas of the piezoelectric ceramic layers 510 sandwiched between the inner center electrodes 530a and the inner side electrodes 540a, 540b, in directions shown by dashed arrows 551, perpendicular to the polarization directions (shown by solid arrows 550). As a result, the two areas in the piezoelectric ceramic layers 510 are deformed symmetrically by a shear effect, and the inner center electrodes 530a are shifted upwardly in FIG. 16, thereby increasing the volume of the ink channel 60a. At this time, ink is supplied from an ink source (not shown) to the ink channel 60. Thereafter, when the application of the drive voltage is stopped, the deformed piezoelectric ceramic layers 510 return to the initial state. Thus, the volume of the ink channel 60a is reduced, and an ink droplet 520 is ejected from the ink channel 60a through a nozzle 50a. 
The piezoelectric ink ejector that incorporates a piezoelectric transducer structured as described above is easy and inexpensive to manufacture and able to accomplish high-resolution printing.
However, in the above-described piezoelectric ink ejector, when the required ink droplet volume and the required ink ejecting velocity are fixed, the required drive voltage is determined by the spaces between the inner center electrodes 530 and the inner side electrodes 540, 540. Thus, the drive voltage cannot be lowered as desired, and the costs of a power source and a driving circuit board will be relatively high. In addition, when the drive voltage is fairly high, the polarization property of the piezoelectric transducer 500 tends to deteriorate due to the drive voltage being applied perpendicularly to the polarization direction, and the lifespan of the ink ejector will be shortened.
If the spaces between the inner center electrodes 530 and the inner side electrodes 540, 540 are lessened to lower the drive voltage, locally deformable areas in the piezoelectric transducer 500 are reduced, and the amount of change in the volume of the ink channel 60 is also reduced. Because of such structural limitations, it is hard to lower the drive voltage.
U.S. Pat. No. 6,174,051 and Japanese Laid-Open Patent Publication No. 10-58675 disclose another piezoelectric transducer, in which a piezoelectric ceramic layer deformable by a piezoelectric longitudinal effect is laminated to the above-described piezoelectric transducer 500 such that the piezoelectric ceramic layers are deformed greatly by a piezoelectric longitudinal effect as well as a piezoelectric shear effect. However, because each layer is deformable by either one of the effects, one layer deformed locally by one of the effect pushes a non-deformed area of another layer, thereby producing a combined deformation in the entire piezoelectric layers. Therefore, a need exists for an improved piezoelectric transducer that is deformed more effectively by a piezoelectric longitudinal effect and a piezoelectric shear effect.